Deathwatch Chaplain
Deathwatch Chaplains like all things Deathwatch, have a slightly different role than the Chaplains of other Space Marine Chapters. While the latter have many different duties within upholding traditions and maintaining morale of their own chapter, the Deathwatch Chaplains' has the hardened duty in maintaining the battle-spirits of not one but multiple brethren from several different chapters. All at once. These guys must have a charisma streak to make Horus Lupercal look as well sophisticated as a Snotling.
Overview
As mentioned, Deathwatch Chaplains need to reform their way of thinking as now they are in command of multiple different brothers from multiple different chapters. This might lead in accidental cultural misunderstandings and the results are often...hilarious.
A Deathwatch Chaplain must study extensively during his training that covers sociological and historical aspects. He must know the beliefs and values of a thousand different chapters and their sometimes contradictory legends of the Primarchs by heart. A Deathwatch Chaplain must become not only a righteous warrior but also a scholar of the Primarchs and of chapter histories originating in the very birth of the Imperium. Thus, when a Deathwatch battle-brother stands at the brink of despair or impotent rage, the Chaplain will know the right liturgies and catechisms to speak, and which chapter heroes or legendary battles of the past to cite that will inspire the warriors of the present. You could say that they're like a bunch of nerds, but the cohesiveness of the Deathwatch is of utmost importance or else the Imperium's most key force of Xeno rapists would fall apart due to different and contradicting beliefs and ways to conduct their Spiritual Liege Codex Astartes.
Deathwatch Chaplains also make it their business to know of every potential motivation, conflict and agenda among their battle-brothers before battle is joined. Many battle-brothers are amazed by the knowledge of Deathwatch Chaplains, and in some cases that knowledge extends even unto the deepest mysteries known only to the inner circle of chapter hierarchies. A Deathwatch Chaplain takes no sides and makes no judgments about individual chapters and in some senses their long study grants them a perspective denied to most. Deathwatch Chaplain doctrines hold that all Space Marines are united in their purpose of service to the Emperor and those bound by their vows to the Deathwatch must be united more than most.
In addition to their traditional garb of black Power Armour and skull-shaped helmet, Deathwatch Chaplains also commonly add many xenos trophies: strips of tanned alien hide, strings of bones, skulls or shrunken heads inscribed with liturgies of hate and remembrance. Their passion for killing aliens is without peer and each trophy will mark a noteworthy foe or memorable battle. A Deathwatch Chaplain is a deep well of tried and trusted techniques for slaying xenos, not only from personal experience but from the vast tracts of Chapter legend and lore that they can draw upon.